Many circuit packages which operate at high frequencies are implemented using multilayer substrates with a relatively small number of layers and high density routing for cost reasons. Performance degradation associated with differential loss and cross-talk is major problem especially prevalent with low layer count substrates with high density routing. Mismatches in impedance between components of the vertical structure carrying differential signals directly leads to differential loss, while cross-talk is strongly related to the separation between differential pairs. For these multilayer substrates, having closely packed differential signal pairs is unavoidable and the close proximity of differential pairs to each other is a major contributor to cross-talk. On the other hand, spacing out pairs of signal traces carrying the differential signals to reduce cross-talk in turn reduces the transition space available to achieve target differential impedance.
A configuration commonly used in low layer count substrates with high density routing has two differential plated through holes (PTHs) and one ground PTH aligned in a triangle configuration. The triangle configuration can not achieve the target differential impedance because of the proximity of the differential pair of PHTs to each other such that differential loss performance is significantly degraded. In addition, conductive material directly above the differential plated through holes or above the differential solder balls may couple with either the PTHs or solder balls. This coupling affects the differential impedance of the signal carried through the PTHs or solder balls.
It is in this context that embodiments of the invention arise.